Complex shape tubular metal products may be manufactured using casting, welding, or pipe hydroforming.
Casting and welding are nearly always labor and energy intensive. In addition, when there are dimensional accuracy and quality standards, these methods require additional finishing and testing operations.
In hydroforming, a hollow tubular workpiece is confined in a die with its inner surface corresponding to the desired outer surface of the part to be produced. Punches or rams are pressed axially against the ends of the workpiece and fluid at great pressure is introduced therein via one of the punches. The combined axial compression of the punches and the internal pressure of the fluid cause the workpiece to expand to fill the die cavity, producing the desired part.
However, radial expansion of a workpiece by applying a high hydraulic pressure to its interior using existing hydroforming methods and equipment, creates tensile stresses in the workpiece material, whereby the resistance to deformation of the workpiece material increases considerably as it is deformed. This is known as work hardening and results in a reduction in plasticity of the workpiece material and in a thinning of the wall of the expanded portion of the workpiece. It can further result in breakage of the wall of the expanded portion beyond a specific limiting expansion ratio. This limits the amount of radial expansion that can be achieved with existing hydroforming methods in a single step of expansion; for example, steels typically are limited to 30-40%, while electrolytic coppers typically are limited to 50-70%. This restricts the variety of products that can be produced without employing multiple hydroforming steps. An additional problem is that the deformation is irregular, resulting in variations in the wall thickness in the part produced, often beyond acceptable standard tolerances.
U.S. Pat. No. 5,097,689 and French patent number 2679159 disclose attempts to improve on the basic hydroforming process by means of a separate hydraulic cylinder (patent FR2679159) or with special rigid inserts (U.S. Pat. No. 5,097,689). To achieve larger radial expansion, intermediate steps of heat treatments are proposed. These, however, add considerably to the complication and cost of the production process. Neither of these patents address the limits to one-step deformation without additional annealing steps or the problem of dimensional accuracy in the basic hydroforming process.